Long range atmospheric signaling devices such as fog horns and the like are federally mandated as aids to navigation in off shore environments where hazardous surface or sub-sea obstacles limit marine navigation.
It is known within the art that by augmenting the transmission of audible sound waves, emanating from a sound source generator, with an acoustical transmission path from the outlet of the driver into the throat of a tube of some selected shape and size and directed out of a mouth located at the end or along the tube, high pitched sounds can be produced that carry for long distances. The frequency response of the horn and its performance is a function of the frequency of sound introduced into the horn, its shape and the length of the sound path. Additionally, the shape of the horn's throat and mouth determine the directional characteristics of the horn as a function of the sound frequency and pressure. Horns are selected to fit particular applications of the sounds being generated. For example, a horn in the form of an elongated cylindrical tube will propagate sounds for which the length of the pipe is equal to an odd number of quarter wavelengths. The frequency response of such a tubular horn features a relatively high amplitude spike at the frequency corresponding to the wavelength that is four times the length of the pipe, and is zero for lower frequencies. The tubular horn will transmit harmonics of the frequency at the spike, but at smaller amplitudes. A tubular horn is therefore suitable for use in propagating sound of a single frequency.
A horn that flares, such as a horn featuring a cross section that increases with distance from the throat of the horn to its mouth, generally has a frequency response that goes to zero for sound frequencies below a cutoff frequency whose wavelength is equal to four times the length of the horn. However, the cutoff frequency tends to flatten for higher sound frequencies and smaller wavelengths. A horn with a constant flare rate, such as an exponential horn in which the cross-sectional area doubles for equal increments of length of the horn, tends to provide a broad, useful bandwidth beyond the cutoff frequency of the horn.
In general, the longest wavelength of sound for which a horn is an effective sound propagator is equal to four times the length of the horn or four times the acoustical transmission path defined by the horn. For sound of shorter wavelengths, the effectiveness of the horn as a sound transmitter depends upon the shape of the horn, that is, dependent upon the flare of the horn and the mouth of the horn. Therefore, the mouth of the horn determines directional characteristics of sound transmitted by a horn.
It is particularly important in the field of acoustical warning signals, such as those produced by sirens and marine foghorns, that the sound must be transmitted over long distances, even though the frequencies of the sounds may be limited. Foghorns generally produce only one or two basic sound frequencies. Further, such warning signals may be concentrated in a selected range of directions. For example, a warning horn set to mark a hazard at sea need not direct sound vertically and can, in fact, concentrate the sound propagation generally horizontally. Also, such a warning horn may limit the horizontal arc through which the sound is propagated, there being no need to direct sound on shore, for example. Further, to maximize the effectiveness of a horn to transmit selected sounds produced by a sound driver, the acoustical transmission path provided by the horn should be tuned to the wavelength of the sounds. Therefore, for a limited horizontal range and a single frequency a relatively short, non-expanding throat may be used with a tuned opening to provide a specific pressure wavelength.
Foghorns and warning horns are often placed in harsh environments, such as on promontories, buoys and marine vessels, where they are subjected to wind, water, and ice. Therefore, it is essential that such horns do not become a funnel for directing water into the throat of the horn and into contact with the driver. It is therefore advantageous to utilize a horn that is configured to be self-draining in a manner whereby the driver or drivers are protected and inaccessible by water condensate and the like.
More specifically the prior art teaches us that foghorns are generally designed to meet various range requirements, for example one-half mile or two-mile models. Typically one-half mile models are known as single emitters where a single transducer (emitter) is used to excite a resonant device. These resonate devices, include acoustical pipe resonators or Helmholtz resonators, which may be used to provide acoustic amplification to the acoustic signal, generated by providing an electrical input to the transducer. Typically, two-mile model designs consist of a plurality of emitters and acoustical resonators arranged in a vertical stack to produce the required sound pressure levels required by federal regulations. It can therefore be understood that the prior art does not teach the need for converting a one-half mile signal to a two mile signal for the same structure and in most cases simply adding more emitters or another emitter/resonator either exceed the federal regulations or tend to cancel each other.
The prior art further fails to teach energy conservation by reducing the electrical load by using a plurality of redundant emitters with a single resonator to achieve a desired frequency.